1. Field of the Invention
The present invention relates to plotting mechanisms having a drawing arm capable of selecting one of a plurality of pens. More specifically, the invention relates to a computer controlled mechanism for sensing that a pen is in the grasp of a drawing arm.
2. The Prior Art
In the present era of automation, numerous devices are being developed that will accurately, efficiently, and quickly perform tasks that just a few years ago were performed manually. A large portion of the reason for automation is the rapid development of computers from the early room-sized devices to the present desk top personal computer. Microprocessor controlled devices such as the personal computer have greatly expanded capabilities for efficiently and accurately processing large amounts of data or other information. However, simply processing the data is not enough; the degree to which a computer is able to effectively convey data and other results of its processing to the user is a key measure of its utility to the user.
In recent years, graphical output has become an increasingly popular and commonly used form of output. For example, with presently available software, such as SPICE, an engineer can simulate complex circuits without ever building them. Rather than presenting the results of such a simulation in a large table of numbers, it is much more desireable and useful to present the output of the simulation in graphical form, such as a plot of output voltage vs. input voltage. This graphical output can be either sent to the screen of a CRT for viewing by the user, or to a plotting mechanism which makes a "hard-copy" of the graph on paper.
As another example of a plotting mechanism, Hewlett-Packard has developed a plotter that can be directly connected to a circuit in the same manner as an engineer could connect an oscilloscope. Thus a trained operator can easily obtain an accurate "hard-copy" of all the information viewable on a oscilloscope, including circuit characteristics such as voltage vs. time or output voltage vs. input voltage.
Plotter mechanisms can also be useful in the business world to provide graphical output such as stock trends against time, or to compare trends in various money markets against time. The output possibilities for plotter mechanisms are limited only by the size of the paper that can be inserted into the particular device. Many existing plotters allow the user to color various portions of his plot with a particular color of his choice. For example, an engineer may wish to compare several functional characteristics on same graph, and for that purpose he may select green for one characteristic, blue for another, and black for another. As another example a stock analyst may find it useful to have separate market trends appear in different colors. For example, the NYSE volume may appear in red, and the Tokyo Stock Exchange volume in blue. This capability for color selection is an important commercial element of virtually all plotters on the market today.
The mechanism for color selection typically includes a rotatable circular carousel having a plurality of pens affixed along its perimeter, each pen being a different color and also being removable from its position on the perimeter. Thus, when a certain pen is to be used for coloring a portion of the plot, a control device, often including a microprocessor, rotates the carousel until the chosen pen is at a transfer point where it can be grasped by a drawing arm. Then the chosen pen may be manipulated by the drawing arm under control of the plotter's control device in order to draw the graph or other figure on paper or any other writing surface.
Color selection occurs automatically as a result of programming, for example, an operator may remotely command the plotter to color the background grid in black, the descriptive writing in blue, and one or more graph lines in green, red, or any other colors. Thus, the graphing process may proceed without operator supervision.
However, a problem arises if at some time in the automatic printing sequence, a pen is non-existent in the selected penholder. In that instance, the drawing arm at the transfer point will attempt to grasp a pen which is non-present, and in fact will grasp nothing. If the plotter is not equipped with a pen sensor of some type, the plotter mechanism will not be aware that a pen is not in the drawing arm, and will continue to direct drawing motions as if a pen were in place. As a result, portions of a graph or other figure may not be drawn. If someone happens to notice that the plotter is attempting to make a drawing without a pen, he or she would probably stop the plotting process, insert a pen in the pen holder, and command the plotter to start again. If the omission is not noticed, the results of failure to draw a portion of a figure may be more serious; the graph or figure might be thought complete and relied upon as such. Costly mistakes or at least substantial embarrassment may be caused by such an error.
Previous pen sensing mechanisms include a variety of systems for detecting the presence of a pen being held by the drawing arm. These mechanisms include optical switches or mechanical switches, each of which is actuable by the existence of a pen in the pen holder. Each switch may be connected to provide an input to a control device, thereby providing direct information to the control device as to the presence or non-presence of a pen.
For example, a mechanical switch may be affixed at a position where the pen (if it is present) will brush against the switch toggle as the drawing arm moves away from the carousel, thereby actuating the switch and providing an output to the control device which is indicative of the presence (or non-presence) of the pen being held by the drawing arm. As an alternate to affixing the switch to the plotter, previous devices have also used a mechanical switch affixed to the drawing arm, so that a pen grasped by the arm will actuate the switch to provide a digital output to the control device.
Alternatively, an optical detection system may be used to detect the pen's presence. One type of such a system uses a LED (Light Emitting Diode), an optical detector, and specially mirrored pens. The light being emitted from the LED bounces off the mirrored pen (if present in the drawing arm) and actuates the optical detector which then provides a signal indicative the pen's presence. Such an optical detection system has a disadvantage because the optical detector is sensitive to ambient light and therefore has an inherent reliability problem; for example, ambient light may actuate the optical detector, falsely indicating the presence of a pen.
Both systems described above, the mechanical switch system and the optical switch system require additional components. These components have a sole, specific purpose, which is to detect the presence or non-presence of a pen. For example, a manufacturer, having selected the switch detection method, must install a switch in the drawing arm or at some location in the plotter mechanism to detect the pen's presence. Different manufacturers who have selected the optical detection method must install a LED, an optical detector, implement a detection algorithm in the control device, and in addition must require the operator to use the specially mirrored pens.
Therefore, both the optical detection system and the mechanical switch system result in additional manufacturing costs.
Furthermore, it is axiomatic that each added component has a probability for failure that is present upon manufacture, and which increases after a time period of use and misuse. Thus, for a manufacturer and for a consumer, the presence of the additional components (the mechanical switch, or the LED and optical detector) introduces a reliability factor in addition to the increased cost.
It would be an advance in the art to provide a method and a device for sensing the presence (or non-presence) of a pen in the drawing arm that does not use specialized components and switches. It would be a further advancement in the art to provide a reliable, low cost, and easily implemented method and device for detecting the presence of a pen being held by a drawing arm.